1. Field of the Invention
The present invention relates to a method of and an apparatus for combining linework data including run-length data, and more particularly to overlay plural linework images one upon another to produced a combined image.
2. Description of the Related Art
Run-length data is a common type of image data to represent a binary image or linework image. Run-length data is used in transferring a linework image through facsimile or storing a linework image in a magnetic disk. Run-length data indicates lengths of "runs" in a linework image for each scanning line. In a black and white image, one "run" means a series of consecutive white pixels or consecutive black pixels on a scanning line, and run-length data indicates the length of each run of the consecutive white pixels or black pixels.
Run-length data is used to represent a colored linework image in commercial printing as well as to represent a black and white image. Each linework is usually colored with the inks of the four primary colors: yellow (Y), magenta (M), cyan (C), and black (K). In order to reproduce a colored linework image, run-length data is prepared for each color of Y, M, C, and K.
In commercial printing, some linework images are combined to form a new linework image. Since the volume of run-length data depends on the number of runs but not directly on the area of the image, the volume of the combined run-length data sometimes increases considerably by the combination. When the capacity of a memory for storing the combined run-length data is not sufficiently large, each linework image before combination is required to be divided into several image areas, and the combination is executed for every divided image area.
Linework data before combination is generally stored in an external storage unit such as a magnetic disk. When combining operation is completed for one of the divided image areas by a combining unit, next linework data is to be supplied from the external storage unit to a memory in the combining unit. Each supply of linework data to the memory in the combining unit stops the combining operation and thereby undesirably extends the executing time of the combining operation.
Incidentally, conventional linework combining process is executed in the order of re-conversion of run-length data to pixel image data; combination of converted pixel image data; and conversion of combined image data into run-length data. The conventional combining process consumes relatively a long time accordingly.